perf: use faster kmean find partition routing for pq assignment

[eddyxu]

• Added benchmark for pq assignment to use fast routing
• Use compute_partition for fast kmeans find partition

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 79.56%. Comparing base (0bf765b) to head (df7b647).

Files	Patch %	Lines
rust/lance-linalg/src/kmeans.rs	71.42%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2515   +/-   ##
=======================================
  Coverage   79.55%   79.56%           
=======================================
  Files         208      208           
  Lines       59348    59351    +3     
  Branches    59348    59351    +3     
=======================================
+ Hits        47216    47220    +4     
- Misses       9371     9373    +2     
+ Partials     2761     2758    -3     

Flag	Coverage Δ
unittests	79.56% <77.77%> (+<0.01%)	⬆️

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[eddyxu]

l2,32768                time:   [254.57 ms 258.13 ms 262.47 ms]
                        change: [-58.154% -57.389% -56.589%] (p = 0.00 < 0.10)
                        Performance has improved.
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) high mild

dot,32768               time:   [251.29 ms 258.31 ms 266.30 ms]
                        change: [-66.756% -65.430% -64.142%] (p = 0.00 < 0.10)
                        Performance has improved.